Secondary, or rechargeable, batteries often include circuitry for identifying the battery type to a charger or host device to which a battery can be coupled. This is advantageous since different types of batteries, such as nickel cadmium batteries or lithium batteries, can require different charging regimes. One commonly used identification device is a coding resistor. A pull-up resistor in the charger applies a bias to the coding resistor in the battery, and the charger, by way of an analog-to-digital (A/D) converter, is able to determine the resistor value by measuring the voltage at the junction of the voltage divider formed by the pull-up resistor in the charger and the coding resistor in the battery. The charger can then identify the type of battery from the resistor value.
A disadvantage of the coding resistor identification system is that resistor tolerance and accuracy of the A/D converter limits the number of different resistors which can be identified by the charger. Typically, about sixteen (16) different battery types can be differentiated using the coding resistor approach.
One method of overcoming the limitations of a coding resistor is to embed a memory device, such as an erasable programmable read only memory (EPROM) in the battery. The EPROM typically contains not only battery type information, but specific information related to the charge regime to be used. In addition, "fuel gauge" information can be stored in the EPROM to enable the host device or charger to accurately determine the state of charge of the battery based on the measured battery voltage. However, some chargers are unable to recognize the information provided by an EPROM, which can cause improper or insufficient charging of the battery.
Thus, what is needed is a battery and charging system in which a greater number of battery types can be identified while providing backward compatibility with existing chargers and batteries.